We have used photoinduced electron transfer (PET) activation of silyl enol ethers for the synthesis of tricyclic hydrocarbons. The mechanism of this reaction was investigated by conducting independent radical-induced cyclizations of corresponding iodo ketones and performing density functional theory (DFT) calculations on the possible intermediates. Our aim was to explain the nature of the reactive intermediate of the cyclization step and to find the causes of the various types of selectivity observed in this process. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.